Cessna 172P hangared in Ontario, Canada
Reiff heater used continuously, with cowl and prop covers.
Graph of temp/humidity in engine cowl, vs. ambient temp (second link)
Engine
http://wirelesstag.net/eth/tempStats.html?54da56f4-95a3-49af-9155-c052537625fc&Engine
Ambient (Hangar door)...